The present invention relates generally to induction heating, and in particular to apparatus for induction heating in which the zero crossing point of high-frequency energization current is detected for controlling the firing angle of gate-controlled switching devices.
The induction heating apparatus usually comprises a gate-controlled switching circuit connected to a source of low frequency alternating current potential, a gating circuit for triggering the switching circuit into conduction, and a commutation circuit including an induction heating coil to provide commutation of current through the conducting switching circuit. The commutation circuit is tuned to a frequency in the inaudible or ultrasonic range, and the pulse repetition rate is usually in the neighborhood of the resonant frequency of the commutation circuit. The current triggered in the commutation circuit is therefore in the inaudible frequency range which is suitable for induction heating purposes.
It is known in the art to control the power level of the apparatus by varying the frequency of the energization current. This frequency control is usually effected by varying the interval between successive trigger pulses in response to the user's setting level.
However, the resonant frequency of the commutation circuit depends largely on the magnitude of inductive coupling between the heating coil and a magnetic work load placed thereover. If the magnitude of inductive coupling widely varies due to a sudden change of loads, there is a likelihood of the occurrence of commutation failure, since the gate-controlled switching circuit fails to turn off prior to the time of occurrence of a subsequent trigger pulse.